The invention relates generally to the field of electromagnetic signal communication and, more particularly, to the filtering of high power signals for broadcast communications.
In the field of broadcast communications, electrical filters are required to separate a desired signal from energy in other bands. These bandpass filters are similar to bandpass filters in other fields. However, unlike most other electrical bandpass filters, filters for broadcast communication must be capable of handling a relatively high input power. For example, a signal input to a broadcast communications filter might have an average power between 5 and 100 kilowatts (kW). Many electronic filters do not have the capacity for such large signal powers.
For many years, high power electrical bandpass filtering has included the use of waveguide cavity filters. In particular, the introduction of dual-mode cavities for microwave filters in 1971 made a significant contribution to the art. Dual-mode filters allowed for a reduction in filter size and mass, and could realize more complex filter functions by their ability to easily couple non-adjacent resonators. Later reductions in size and mass were achieved with the introduction of triple and quadruple mode filters.
While dual-mode waveguide cavity filters have been used often for space and satellite communications, they have also been used for terrestrial television broadcast applications. Indeed, for transmitters operating in a common amplification mode (i.e., a mode in which both audio and video signals are being amplified together), dual-mode filters have become predominant because of their low loss and ability to realize complex filter functions. Moreover, dual-mode filters have been favored for the transmission of analog television signals because of their flexibility in realizing wide pass bandwidths to compensate for frequency drift due to RF heating and ambient temperature changes. However, with the advent of digital television, system requirements have changed. The FCC emissions mask for digital television broadcast stations is very restrictive for power radiated into adjacent channels or out-of-band frequencies. These requirements will not be satisfied by filters that have wide passbands that are allowed to drift.
In the past, waveguide cavities have been developed that are adjustable to compensate for thermal expansion. Paul Goud in Cavity Frequency Stabilization with Compound Tuning Mechanisms, Microwave Journal, March 1971 discloses a waveguide cavity that may be adjusted to compensate for thermal expansion. In FIG. 2 of the article, Goud shows a compound tuning mechanism that may be used to change the effective length of the filter cavity. However, this tuning mechanism requires a manual adjustment of a screw device to make the necessary changes. Moreover, the movable surface is based on a two-section choke. This choke must be unconnected to the sides of the filter, so that it may be moved relative to them. As such, the cavity is unsealed, and is prone to leakage and poorer performance than a sealed filter.
More recently, filter design has addressed the need for narrower bandwidth filters by constructing filters from materials with lower thermal expansion coefficients to minimize the effect of heating on the filter dimensions. In particular, the nickel/steel alloy Invar(copyright) (a registered trademark of Imphy, S.A., Paris, France) has been used as a cavity material. Because of its extremely low degree of thermal expansion, the cavities built with Invar(copyright) suffer less of a dimensional change with heating, and therefore maintain a narrower, more stable passband. However, Invar is also very expensive, and consequently drives up the overall cost of the filter.
In accordance with the present invention, a bandpass filter is provided that uses the deformation of a cavity surface in response to thermal changes to compensate for the resonant frequency shifting effects of thermal expansion. The filter has at least one waveguide cavity in which an input electrical signal resonates at a desired resonant frequency, and a plurality of surfaces, each with a predetermined geometric shape. For example, in a preferred embodiment, the filter has a cylindrical outer surface and a circular end plate. A thermal compensator is provided that responds to thermally induced changes in dimensions of the cavity by distorting the shape of one of the cavity surfaces, thereby minimizing any resulting drift in the resonant frequency.
Typically, the thermally induced changes in the cavity are an increase in cavity dimensions, and the thermal compensator deflects one of the cavity surfaces inward, such as in the case of a concave deflection of the cavity end plate. In the preferred embodiment, the thermal compensator includes a control rod that limits the movement of at least a first point on an end plate of the cavity in a first direction. That is, the control rod prevents movement of that point in the direction of thermal expansion. Thus, as the cavity expands, outer portions of the end plate move in the direction of the expansion, but the first point is restricted by the control rod. As a result, the end plate is deformed from its original shape. The control rod has a coefficient of thermal expansion that is significantly different (typically lower) than that of a material from which the cavity is constructed.
In one embodiment, the control rod fixes a point on the cavity end plate relative to a different location on the filter. This different location may be such that the control rod spans more thermally expanding material than that which defines the waveguide cavity. In such a case, the thermal expansion causes the point of deflection to be moved relative to its original position. In other words, whereas the deflection point initially resides in a first plane perpendicular to the direction of thermal expansion, the expansion of the thermally expanding material spanned by the control rod forces the deflection point out of its original plane toward an interior of the cavity. In another embodiment, a similar inward movement of the deflection point may be accomplished by using an end deflecting rod that connects the control rod to the deflection point. If the end deflecting rod has a coefficient of thermal expansion that is significantly higher than that of the control rod, its expansion will force the deflection point inward relative to the control rod. Naturally, these two techniques may also be combined.
In determining the appropriate amount that a cavity surface point should be deflected, a theoretical model may be used to first establish how far a movable end plate would have to be moved to compensate for an expansion of the waveguide cavity without the end plate being distorted. The resulting deflection distance may then be augmented to compensate for the fact that, in the present invention, the entire surface is not being moved. This additional deflection may be determined empirically, and can provide a more accurate compensation for control of the cavity resonant frequency.
In a preferred embodiment, the waveguide cavity is one of two cavities, which are coupled together via an iris plate. Each of the cavities may be thermally compensated in the manner described herein. One particularly preferred embodiment is a six section filter consisting of two thermally compensated waveguide cavities, each with two orthogonal resonant modes, and two coaxial resonators, each coupled to one of the waveguide cavities via an impedance inverter. The signal to be filtered is input through one of the coaxial resonators to one of the waveguide cavities and output through the other coaxial resonator.
In an alternative embodiment, control rods are not used but, instead, a surface of each waveguide cavity is fixed relative to the remainder of the cavity, and has an inner surface shaped to counteract any increase in cavity volume resulting from thermal expansion. In particular, an end plate of the cavity may be given a convex shape, with the convex portion extending away from the interior of the cavity. That is, the end plate may be xe2x80x9cdome-shaped,xe2x80x9d and secured along its periphery to as main portion of the cavity. Relative to a plane in which the end plate is connected to the remainder of the cavity, its convexity projects away from an interior of the cavity. As temperature increases cause thermal expansion of the cavity in a radial direction, the end plate experiences stress in directions perpendicular to an axis of symmetry of its convexity. It is thereby pulled radially outward, causing the convex portion of the end plate to be forced to a flatter profile, deflecting toward the interior of the cavity. As a result, the change in shape of the end plate tends to counter any volume expansion that would otherwise result from the thermal expansion of the cavity, and inhibits corresponding shifts in the filter frequency response.